Lug setback? What am I missing?
- Thread starter Mram10us
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Alex Wheeler
Well-Known Member
6br with a 55 grain bullet vs a 30-06 with 200 grain bullet. Taking chamber wall grip out of it, both will show equal bolt thrust and stiff bolt lift at the same pressure. But obviously will have large differences in recoil.
Stiff bolt lift can be caused by temporary lug setback. The bolt and lug seats compress enough to allow the case brass to yield. With actions that have larger lug seats you may never experience stiff bolt lift. With these actions we have to measure case head expansion because they dont have that temporary lug setback and you will drop a primer out of the case before getting a stiff bolt.
Stiff bolt lift can be caused by temporary lug setback. The bolt and lug seats compress enough to allow the case brass to yield. With actions that have larger lug seats you may never experience stiff bolt lift. With these actions we have to measure case head expansion because they dont have that temporary lug setback and you will drop a primer out of the case before getting a stiff bolt.
Alex Wheeler
Well-Known Member
Might be easier to talk
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Great explanation. I'm guessing I wasn't clear on the definition. So would an easy to understand definition be the "initial pressure in the chamber causing the projectile to begin moving"? In other words, the lug setback occurs before recoil is felt or is the beginning stage? Thanks for taking the time
bigngreen
Well-Known Member
Peak pressure is roughly at 6 milli seconds from the primer strike from what I've seen from pressure transducers, peak pressure is peak bolt thrust too.
Alex Wheeler
Well-Known Member
Peak chamber pressure happens very early and the bullet is actually out of the barrel long before you feel recoil.
I get that, just was confused on the definition. I assumed the bolt lug setback was the total energy felt from the projectile. I was surprised to see on a graph the psi is around 1k at the muzzle of a 26" barrel. That led to playing with aluminum muzzle brakes 
again, thanks
again, thanksbigngreen
Well-Known Member
Muzzle pressure is all over the board, many will be in the 8 k range, I've seen 13k, my bro wrote a spread sheet for me that I could take exit pressure from quick load or a pressure transducer and plug it in and it would calcululate if I was in a safe range for a given muzzle brake thread, cal and material.
Johnny Jr.
Member
Mram10 us.... thanks for starting this thread. At first, I was dumbfounded that people were trying to say recoil has NOTHING to do with forces on the bolt. To me it was obvious that recoil from muzzle energy had to get to my shoulder mostly through the bolt lugs. I understood the pressure and cartridge head size formula to figure bolt forces....but certainly the foot pounds of energy causing recoil had to be a force ADDED to the bolt.....right?
I wanted to take exception to the Wiki article on bolt forces because it clearly only calculates PSI x inside case head diameter. It ignores energy related to recoil (bullet weight, speed, and mass of the gun powder).
I don't think Wiki is the arbiter of all truth in the universe, but I decided to NOT refute the formula that calculates bolt forces and then try to wrap my mind around how the recoil energy bypasses the lugs on its way to the gun butt.
The response to the Q and A in post #175 is what made the light bulb come on. My best attempt to explain it is: (And I'm not talking down to you because I know you know the basics as well or better than I did at the start) There is an equal and opposite reaction of the bullet mass being pushed through the barrel. The only thing the bullet has to push against on its way out is the grooves and lands of the tube it is in. So as the little mass is going one way, it's pushing back on the big mass. The tube is being shoved back into the receiver. The receiver is locked down to the stock by the recoil lug and action screws and in turn to your own body mass. So basically, that entire mass is pushing against the bullet........but here's the key (to me, anyway).....it's all going through the lands and grooves......not the bolt or lugs.
I completely agree with you that exaggerating the variables makes it easier to grasp. That's why I had such problem with the example of 8000 and 4300 ft lb in post 175 having the same bolt forces, yet NO recoil going through the bolt.
As a side note, my wife reminds me every day that I'm dumber than I think. But at least I learned something because you started this thread.Thanks.
Well said, Johnny. It still seems like there is more to it, but I need to experiment. I respect Alex opinion because he knows a lot about rifles yet doesn't throw it in your face. Those are the best teachers The definition is still eluding me to a point, so I need to read up more. I'm trying to apply principles I've learned in past lives.... which are probably totally different. Either way, a lot of good information has come from people. Definitely shows the knowledge base of our folks
The definition is still eluding me to a point, so I need to read up more. I'm trying to apply principles I've learned in past lives.... which are probably totally different. Either way, a lot of good information has come from people. Definitely shows the knowledge base of our folks
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I ran across this thread today looking for something else. Might help put some new life in this thread.
https://www.longrangehunting.com/threads/500-jeffery-based-wildcats.107936/page-3 Look at post #31.
https://www.longrangehunting.com/threads/500-jeffery-based-wildcats.107936/page-3 Look at post #31.
Gwine
Well-Known Member
Recoil is the rifle moving in the opposite direction of the rapidly accelerating projectile and gasses escaping the barrel.
The lugs/abutments are solely there to contain the explosion happening in the chamber. Pressure trying to escape in every direction.
I can only imagine the lugs/abutment experiencing recoil energy if you lubricated your cartridge and set an excessive headspace. (Bad)
The lugs/abutments are solely there to contain the explosion happening in the chamber. Pressure trying to escape in every direction.
I can only imagine the lugs/abutment experiencing recoil energy if you lubricated your cartridge and set an excessive headspace. (Bad)
Imo, The lugs must get recoil forces.
What is the bullet pushing off of? Boltface?
What does the boltface push off of? Lugs?
What do the lugs lush off of? Lug abatements?
How can they receive NO energy?
Great Find, Sir! Thanks
